Localize S18 and S28 rRNA within the S45 precursor RNA of mouse L and HeLa cellsby electronmicroscopy. Use various methods of visualizing 3'-termini of RNA's. We shall not use the 3'-exonuclease shortening of the RNA molecules since endonuclease contaminations are likely to falsify the identification of the termini. Study the UV effects on transcription of HnRNA in mouse L and HeLa cells. In particular, investigate the applicability of the UV mapping technique to determinations of the transcriptional linkage of select mRNA species.